1. Field of the Invention
The present invention generally relates to an outboard drive with a speed change mechanism (e.g., a transmission) and, more particularly, to an outboard drive having a speed change mechanism for transmitting a rotational speed different from an output speed of a prime mover.
2. Description of Related Art
Outboard drives are coupled with an associated watercraft to propel the watercraft forward or backward. Outboard motors are typical one kind of such outboard drives.
Typically, outboard motors include a drive unit and a mount unit. The mount unit can be fixed to a transom board for mounting the drive unit on an associated watercraft. The drive unit includes an engine at the top thereof as a prime mover. The engine has a crankshaft that generally extends vertically as an output shaft of the engine. A driveshaft is coupled with the crankshaft and extends downward from the crankshaft. A propulsion shaft is coupled with the driveshaft and extends generally normal to the driveshaft. Typically, a propeller is coupled with the driveshaft. A casing normally houses the driveshaft and the propeller shaft.
The crankshaft rotates in the engine. The rotation of the crankshaft is transmitted to the propeller through the driveshaft and the propeller shaft. The propeller thus rotates to generate thrust for propelling the watercraft.
Some outboard motors include a speed change mechanism or transmission that changes a rotational speed of the crankshaft to a different speed and transmits the changed speed to the propulsion shaft. For example, Japanese Patent No. 2785200 discloses an outboard motor having a planetary gearing mechanism as the speed change mechanism. The driveshaft of this outboard motor is divided into an upper portion and a lower portion. The planetary gearing mechanism is interposed between the upper and lower portions of the driveshaft. Thus, at least a mid portion of the casing, which encloses the driveshaft, needs to have a larger size to enclose the planetary gearing mechanism. That is, the casing must be larger, at least in this portion, which spoils the external appearance of the outboard motor. On the other hand, if the whole casing is enlarged to improve the external appearance, the submerged portion of the casing will produce more drag as the result of its enlarged size.